1. Field of the Invention
This invention relates generally to the production of single-wall nanotubes; in particular, to gas-phase nucleation and growth of single-wall carbon nanotubes from high pressure CO.
2. Description of Related Art
Fullerenes are closed-cage molecules composed entirely of sp2-hybridized carbons, arranged in hexagons and pentagons. Fullerenes (e.g., C60) were first identified as closed spheroidal cages produced by condensation from vaporized carbon.
Fullerene tubes are produced in carbon deposits on the cathode in carbon arc methods of producing spheroidal fullerenes from vaporized carbon. Ebbesen et al. (Ebbesen I), “Large-Scale Synthesis Nanotubes,” Nature, Vol. 358, p. 220 (Jul. 16, 1992) and Ebbesen et al., (Ebbesen II), “Carbon Nanotubes,” Annual Review of Materials Science, Vol. 24, p. 235 (1994). Such tubes are referred to herein as carbon nanotubes. Many of the carbon nanotubes made by these processes were multi-wall nanotubes, i.e., the carbon nanotubes resembled concentric cylinders. Carbon nanotubes having up to seven walls have been described in the prior art. Ebbesen II; Iijima et al., “Helical Microtubules Of Graphitic Carbon,” Nature, Vol. 354, p. 56 (Nov. 7, 1991).
Single-wall carbon nanotubes have been made in a DC arc discharge apparatus of the type used in fullerene production by simultaneously evaporating carbon and a small percentage of Group VIII transition metal from the anode of the arc discharge apparatus. See Iijima et al., “Single-Shell Carbon Nanotubes of 1 nm Diameter,” Nature, Vol. 363, p. 603 (1993); Bethune et al., “Cobalt Catalyzed Growth of Carbon Nanotubes with Single Atomic Layer Walls,” Nature, Vol. 363, p. 605 (1993); Ajayan et al., “Growth Morphologies During Cobalt Catalyzed Single-Shell Carbon Nanotube Synthesis,” Chem. Phys. Lett., Vol. 215, p. 509 (1993); Zhou et al., “Single-Walled Carbon Nanotubes Growing Radially From YC2 Particles,” Appl. Phys. Lett., Vol. 65, p. 1593 (1994); Seraphin et al., “Single-Walled Tubes and Encapsulation of Nanocrystals Into Carbon Clusters,” Electrochem. Soc., Vol. 142, p. 290 (1995); Saito et al., “Carbon Nanocapsules Encaging Metals and Carbides,” J. Phys. Chem. Solids, Vol. 54, p. 1849 (1993); Saito et al., “Extrusion of Single-Wall Carbon Nanotubes Via Formation of Small Particles Condensed Near an Evaporation Source,” Chem. Phys. Lett., Vol. 236, p. 419 (1995). It is also known that the use of mixtures of such transition metals can significantly enhance the yield of single-wall carbon nanotubes in the arc discharge apparatus. See Lambert et al., “Improving Conditions Toward Isolating Single-Shell Carbon Nanotubes,” Chem. Phys. Lett., Vol, 226, p. 364 (1994). High quality single-wall carbon nanotubes have also been generated by arc evaporation of a graphite rod doped with Y and Ni. See C. Journet et al., Nature 388 (1997) 756, hereby incorporated by reference in its entirety. These techniques allow production of only gram quantities of single-wall carbon nanotubes at low yield of nanotubes and the tubes exhibit significant variations in structure and size between individual tubes in the mixture.
An improved method of producing single-wall nanotubes is described in U.S. patent application Ser. No. 08/687,665, entitled “Ropes of Single-Walled Carbon Nanotubes” incorporated herein by reference in its entirety. This method uses, inter alia, laser vaporization of a graphite substrate doped with transition metal atoms, preferably nickel, cobalt, or a mixture thereof, to produce single-wall carbon nanotubes in yields of at least 50% of the condensed carbon. See A. Thess et al., Science 273 (1996) 483; T. Guo., P. Nikolaev, A. Thess, D. T. Colbert, R. E. Smalley, Chem. Phys. Lett., 243, 49–54 (1995), both incorporated herein by reference. The single-wall nanotubes produced by this method tend to be formed in clusters, termed “ropes,” of 10 to 1000 single-wall carbon nanotubes in parallel alignment, held together by van der Waals forces in a closely packed triangular lattice. Nanotubes produced by this method vary in structure, although one structure tends to predominate. These high quality samples have for the first time enabled experimental confirmation of the structurally dependent properties predicted for carbon nanotubes. See J. W. G. Wildoer, L. C. Venema, A. G. Rinzler, R. E. Smalley, C. Dekker, Nature, 391 (1998) 59; T. W. Odom, J. L. Huang, P. Kim, C. M. Lieber, Nature, 391 (1998) 62. Although the laser vaporization process produces improved single-wall nanotube preparations, the product is still heterogeneous, and the nanotubes are too tangled for many potential uses of these materials. In addition, the vaporization of carbon is a high energy process and is inherently costly.
Another known way to synthesize nanotubes is by catalytic decomposition of a carbon-containing gas by nanometer-scale metal particles supported on a substrate. The carbon feedstock molecules decompose on the particle surface, and the resulting carbon atoms then diffuse through the particle and precipitate as a part of nanotube from one side of the particle. This procedure typically produces imperfect multiwalled nanotubes in high yield. See C. E. Snyder et al., Int. Pat. WO 9/07163 (1989), hereby incorporated by reference in its entirety.
Yet another method for production of single-wall carbon nanotubes involves the disproportionation of CO to form single-wall carbon nanotubes+CO2 on alumina-supported transition metal particles such as Mo, Mo/Fe, and Ni/Co. See Dai, H. J. et al., “Single-Wall Nanotubes Produced by Metal-Catalyzed Disproportionation of Carbon Monoxide,” Chem. Phys. Lett., 1996. 260 (3–4): p. 471–475. In this process the transition metal particles on the alumina support that were large enough to produce multi-walled nanotubes were preferentially deactivated by formation of a graphitic overcoating, leaving the smaller metal particles to catalyze the growth of single-wall carbon nanotubes. Good quality single-wall carbon nanotubes can be grown from alumina-supported catalysts even with hydrocarbon feed stocks such as ethylene, provided the multi-walled nanotube production is suppressed by a pretreatment process. See Hafner, H. F. et al., “Catalytic Growth of Single-Wall Carbon Nanotubes From Metal Particles,” Chem. Phys. Lett., 1998. 296 (1–2): p. 195–202; and U.S. Provisional Patent Application No. 60/101,093, entitled “Catalytic Growth of Single Wall Carbon Nanotubes from Metal Particles,” and International Application No. PCT/US99/21367, hereby incorporated by reference in their entirety. These methods use cheap feed stocks in a moderate temperature process. Their yield is intrinsically limited due to rapid surrounding of the catalyst particles and the alumina particle that supports them by a dense tangle of single-wall carbon nanotubes. This tangle acts as a barrier to diffusion of the feedstock gas to the catalyst surface, inhibiting further nanotube growth. Removal of the underlying alumina support from the nanotubes that form around it will be an expensive process step.
Hollow carbon fibers that resemble multi-walled carbon nanotubes have been produced from entirely gas phase precursors for several decades. See Dresselhaus, M. S., G. Dresselhaus, and P. C. Ecklund, Science of Fullerenes and Carbon Nanotubes, 1996, San Diego: Academic Press, 985. Endo first used ferrocene and benzene vapors traveling through a quartz tube in an Ar+H2 carrier gas at about 1000° C. to make carbon nanotubes (imperfect multi-walled carbon nanotubes) overcoated in a largely amorphous carbon. See Endo, M., “Grow carbon fibers in the vapor phase,” Chemtech, 1988: p. 568–576. Tibbetts has used ferrocene and iron pentacarbonyl to produce similar hollow carbon fibers from methane/hydrogen mixtures at 1000° C., a process that he finds is benefited by the addition of sulfur in the form of H2S. See Tibbetts, G. G., “Vapor-Grown Carbon Fibers: Status and Prospects. Carbon,” 1989. 27(5): p. 745–747. In some of Endo's early experiments it is clear that small amounts of single-wall carbon nanotubes were produced as well. But until recently no means has been found to adapt these gas phase methods to produce primarily single-wall carbon nanotubes.
Very recently it has been found that control of the ferrocene/benzene partial pressures and addition of thiophene as a catalyst promoter in the all gas-phase process can produce single-wall carbon nanotubes. See Sen, R. et al., “Carbon Nanotubes By the Metallocene Route,” Chem. Phys. Lett., 1997 267(3–4): p. 276–280; Cheng, H. M. et al., “Large-Scale and Low-Cost Syntheses of Single-Wall Carbon Nanotubes By the Catalytic Pyrolysis of Hydrocarbons,” Appl. Phys. Lett., 1998. 72(25): p. 3282–3284; Dresselhaus, M. S., “Carbon Nanotubes—Introduction,” Journal of Materials Research, 1998. 13(9): p. 2355–2356. However, all these methods involving hydrocarbon feed stocks suffer unavoidably from the simultaneous production of multi-walled carbon nanotubes, amorphous carbon, and other products of hydrocarbon pyrolysis under the high temperature growth conditions necessary to produce high quality single-wall carbon nanotubes.
Therefore, there remains a need for improved methods of producing singlewall nanotubes of greater purity and homogeneity.